Silver (Ag) has such advantages as excellent conductivity, conductivity of heat, weldability, low contact resistance and high reflectivity in visible light region. Therefore, use of Ag and Ag alloy which have extremely high reflectivity in visible light region, can greatly improve the luminous efficiency of LED for preparation of reflector. However, Ag is vulnerable to electromigration. In LED, the chip side is a p-n interface area, and its potential gradient is higher than other areas. Therefore, Ag tends to migrate towards the p-n interface of chip side, resulting in gradient distribution path, which greatly influences LED efficiency and stability. Under extra electric field and corresponding humidity sensing environment (whether in metal or other alloy status, e.g., CuAg, AgPd or SnAg) will transfer to Ag+ ion and generate another metallic silver area after original structure migration of surface material and deposition. This metallic silver area, if on the surface of an electronic device, will grow into dendritic or “whisker-shaped” conductive path, resulting in short circuit between the electrons.
In order to prevent Ag diffusion and electromigration, in general, one or several barrier layers cover on the reflecting layer to block Ag migration (see, e.g., U.S. Pat. No. 6,194,743B1, the disclosure of which is hereby incorporate by reference in its entirety). FIG. 1 illustrates a cross-sectional view of such an LED, including a sapphire substrate 2, an n-layer 3, an active layer 4, and a p-layer 5. An n-electrode 7 is provided in the conventional manner. Silver layer 101 is covered with a diffusion barrier layer 102. Diffusion barrier layer 102 is covered by the bonding layer 103 to which wire or other bonding connections are made. However, the barrier effect is not satisfactory as the Ag is easily diffused and generates electromigration at the chip edge, resulting in chip failure.